Project Summary Physiological homeostasis is essential for maintenance of ocular lens transparency and accommodation, the dysregulation of which can lead to cataract formation and impaired vision. Despite continued efforts, a thorough understanding of the molecular basis of ionic and osmotic homeostasis within the lens is lacking. Owing to this gap in knowledge, we are yet to identify therapeutic avenues to prevent or delay cataract formation, a leading cause of blindness worldwide. In this exploratory application, our broad objective is to investigate and understand the role of calcium binding protein, S100A4, in lens integrity and function. Deficiency of S100A4 leads to late onset cortical cataract formation in mice. Motivation for this proposal stems from our recent novel observation that S100A4, a small molecular weight calcium binding protein involved in cell motility, differentiation and osmoprotection, exhibits a rather discrete and intense distribution pattern in differentiating lens fibers within the eye. Most significantly, absence of S100A4 leads to cortical cataract formation in mice, with progressive histological changes in lens fiber cell morphology and organization presenting by six months of age. Interestingly, hyperosmotic stress also significantly elevated the levels of S100A4 in normal mouse lens. Although S100A4 has been studied extensively in metastasis of various cancers and is considered a prognostic marker and therapeutic target for different cancers, the physiological role of this protein is largely unknown. Therefore, this application not only addresses the role of S100A4 in lens osmotic and calcium homeostasis and lens function for the first time, but also leverages the lens as a model system to elucidate the physiological role of S100A4. Since nothing is known about S100A4 function in lens and cataractogenesis, this developmental proposal seeks to gain molecular and cellular insights into how deficiency of S100A4 accelerates cortical cataract formation, under two specific aims. Aim 1 will investigate the cellular and molecular basis underlying the requirement of S100A4 for lens transparency by performing detailed histological, biochemical, cytoskeletal and gene expression analyses of S100A4 null mouse lenses during maturation and aging. Aim 2 will test the hypothesis that elevated levels of intracellular calcium, calcium-dependent protease activity and impaired osmoadaptation are causative factors in cortical cataract formation in S100A4 null mice. Our expectation is that successful completion of the studies described in this exploratory proposal will have a significant impact on: 1.Deciphering the physiological role of S100A4, 2. Fundamental molecular mechanisms involved in lens homeostasis, and 3. Unraveling the etiological mechanisms of late onset cortical cataract formation. Importantly, we envision that the data derived from this developmental proposal will lay a strong foundation for a comprehensive future project addressing the role of S100A4 in lens integrity, function and cataractogenesis.